The present invention relates to a semiconductor device and, more particularly, to a floating-gate EPROM (Electrically Programmable Read Only Memory) whose contents are erasable with UV (Ultra-Violet) radiation (the semiconductor device hereinafter referred to as a "UV-EPROM").
As the memory capacity of a UV-EPROM is increased, it is desirable to it with a redundant circuit, similar to a random access memory with a large memory capacity. The redundant circuit requires storage cells for storing redundant control information used for detecting address data, which designates defective memory cell, order to interchange the defective memory cell with a redundant memory cell. In the UV-EPROM, a floating-gate PROM (Programable Read Only Memory) cell is widely used as the redundant control information storage cells. Floating-gate PROMS have the same structure as EPROM memory cells, because it is convenient to electrically program the redundant control information. However, the redundant control information must not be erased with UV radiation. Therefore, the PROM cell in the redundant circuit can not receive UV radiation.
For this purpose, a metal shield layer is fabricated to cover and surround the PROM cell. However, the metal shield layer cannot completely cover and surround the PROM cell, because interconnection lines extend from the gate and drain thereof so that the redundant control information can be programmed therein. The metal shield layer is isolated from the interconnection lines by an insulating film such as a silicon dioxide film, and consequently the UV radiation or light must first propagate through the insulating film to reach the PROM cell. Hence the redundant control information stored in the PROM cell is only slowly erased.
If only small amounts of radiation UV reaches the PROM cell in the redundant circuit, it takes a long time to erase the redundant control information. In other words, a floating-gate type PROM cell, which is hardly erasable with UV light, is required. For this purpose, prior art teaches forming the metal shield layer over a wide area along the interconnection lines leading from the gate and drain of the PROM cell. As a result, most UV light entering the insulating layer is absorbed when passing therethrough, and only small amounts of UV light can reach the PROM cell. However, this means that the redundant circuit occupies a large area on a semiconductor chip.